This invention relates generally to telecommunications apparatus and, in particular, to radiotelephones that are compatible with a spread spectrum (SS), code division, multiple access (CDMA) protocol.
A direct sequence code division spread spectrum communication technique in essence combines two digital signals, or bit streams, to create a third signal prior to transmission. The first signal is an information signal, such as the output of a digitized voice circuit. For example, the first signal may have a bit rate of 10 kb/s. The second signal is generated by a random-sequence, or pseudonoise (PN) generator, and is a stream of essentially random bits having a bit rate that can be several orders of magnitude greater than the bit rate of the digitized voice signal. The modulation of these two signals results in the third signal having the same bit rate as the second signal. However, the third signal also contains the digitized voice signal. When transmitted the information carrying signal is spread over substantially the entire bandwidth (e.g., 1.23 MHz) of the transmission channel. At the receiver an identical random-sequence generator produces a random bit stream which mirrors the original random-sequence that was used for modulation at the transmitter. For proper operation, after carrier frequency demodulation, the PN generator of the receiver must be synchronized to the incoming PN sequence. By removing the random sequence from the received signal and integrating it over a symbol period, a despread signal is obtained. Ideally, the despread signal exactly represents the original digitized voice signal.
In the U.S. digital cellular and PCS systems there are multiple frequency bands, each of which contains multiple channels. Any of these channels can be assigned to different systems; such as CDMA, AMPS or IS-54 (TDMA) in cellular; and CDMA, GSM (TDMA) or IS-136 (also TDMA) in PCS.
When a CDMA mobile station powers on outside of its home area (i.e., is roaming), it must search a number of these channels to find a CDMA channel. However, this can require a significant amount of time, particularly if the list of channels to be searched is large. For example, the time required to search the full PN code space looking for a CDMA signal (e.g. a pilot channel PN sequence) can be as much as 2.5 seconds. The system synchronization or search time can thus be substantial when multiple channels must be examined.
At present a CDMA system specification is being drafted for System Selection for Preferred Roaming (SSPR), referred to as IS-683. This specification improves the system selection procedure by requiring mobile stations to maintain a Preferred Roaming List of channels/SIDs (system identifications). Certain CDMA cellular system operators are also drafting their own specifications for SSPR to optimize the system selection process for their networks. However, in all of these new system selection techniques, as presently understood, the Preferred Roaming List that must be maintained by the mobile station can be large, resulting in possibly excessively long system access times (e.g. minutes). As may be appreciated, many users may find it objectionable to wait some number of minutes after turning on a mobile station before obtaining a CDMA system acquisition and being able to place a call.
It is a first object and advantage of this invention to provide an improved system selection procedure for a mobile station.
It is a second object and advantage of this invention to provide a reduced system acquisition time for a roaming mobile station.
It is a further object and advantage of this invention to provide a technique to rapidly prioritize RF channels during a system acquisition procedure, by identifying RF channels having one or more characteristics that are indicative of the RF channel being associated with a desired type of wireless communications system, or conversely by identifying RF channels having one or more characteristics that are indicative of the RF channel being associated with a non-desired type of system.
It is another object and advantage of this invention to provide a significant increase in the speed of synchronization (system selection) of a roaming CDMA mobile station, by prioritizing channel searches based on a spectral analysis of the channels.
The foregoing and other problems are overcome and the objects and advantages are realized by system search methods and apparatus in accordance with embodiments of this invention.
This invention has grown out of a realization by the inventor that if RF channels that contain signals from other, non-desired, systems are assigned a low priority during the system search procedure, then the average acquisition time, when roaming, can be significantly reduced.
An aspect of this invention is a significant increase in the speed of synchronization (system selection) of a CDMA mobile station when roaming, by prioritizing channel searches based on a spectral analysis of channels. The spectral analysis is used to recognize if a particular channel contains a signal other than a CDMA signal (e.g. contains instead a TDMA or a FDMA signal). This information is then used to determine which channels are more likely to be CDMA channels, and to search these channels first (i.e., with a higher priority), or to only search these channels.
In performing the spectral analysis a receiver automatic gain control (AGC) signal is preferably first allowed to settle to a steady state value. Further in accordance with this invention the level to which the AGC signals settles may be used as additional information as to whether there is a CDMA signal present in the channel being spectrally analyzed.
The time required to perform the spectral analysis (e.g., a FFT analysis) of a particular channel is at least an order of magnitude less than the time required to search the full PN code space for a CDMA signal (e.g., 50 mS as compared to 2.5 secs). The use of the teaching of this invention is thus very beneficial in a PCS type of CDMA mobile station, where there are a large number of bands/channels to search when roaming.
The teachings of this invention improves over conventional techniques by using the spectral analysis information to prioritize the channels to be searched. A signal recognition function assigns each channel a rating or search priority, depending on the result of the spectral analysis which may be used alone, or in combination with one or more other indicators, such as the level of the receiver AGC.
In accordance with a presently preferred embodiment a method is disclosed for operating a CDMA mobile station that stores a Preferred Roaming List, as is a mobile station that operates in accordance with the method. The method includes steps of (a) tuning a receiver to an RF channel identified in the Preferred Roaming List; (b) allowing an AGC signal to be generated and to settle; (c) sampling and digitizing outputs of In-phase and Quadrature-phase signal channels, and storing the digitized samples; (d) performing a FFT spectral analysis using the stored digitized samples; (e) based at least on a result of the spectral analysis and a value of the AGC signal, determining a likelihood that the RF channel is a CDMA channel; and (f) storing an indication of the likelihood that the RF channel is CDMA channel for use during a system synchronization and acquisition task.
Using the technique of this invention those channels that may contain AMPS and TDMA channels, such as GSM channels, can be identified and marked such that they are not later PN searched, or are PN searched with a lower priority than the channels identified as likely being CDMA channels.